Referring to FIG. 1, there is shown an electronic tuner for CATV, as an example, which consists of a VHF band section 1 for receiving and amplifying a VHF band signal, a super band section 2 for receiving a super band signal to convert it into an intermediate frequency signal and to feed the resultant signal to the succeeding stage, a UHF band section 3 for receiving a UHF band signal to convert it into an intermediate frequency signal and to supply the converted signal to the succeeding stage, a VHF band local oscillator 4, a VHF band mixer stage 5, etc.
The VHF band section 1 acts to selectively amplify high or low channels of VHF, and it performs a tuning operation by altering the voltage applied to a tuning voltage terminal 11 and altering the capacitance of a variable capacitance diode in the section. Thus, if a voltage is applied to a VHF high band terminal 12, it will be possible to receive a VHF high band signal, and if a voltage is applied to a VHF low band terminal 13, a VHF low band can be received. In any case, when a voltage is applied to either the high band terminal 12 or low band terminal 13, the section 1 operates, and a signal selected and amplified thereby is fed to the VHF band mixer stage through a coupling capacitor 14. At this time, the local oscillator 4 operates and injects a local oscillation signal into the mixer stage 5 through an injection capacitor 41. Consequently, the mixer stage 5 acts as a mixer to produce an intermediate frequency signal at its output terminal 53.
Meanwhile, if a voltage is applied to a super band terminal 21, the super band section 2 operates. Thus, a voltage is applied to a switching diode 22 and a current flows through a resistor 23 and so the diode 22 is triggered into conduction. As a result, an intermediate frequency signal, which is produced as the section 2 receives the super band signal, is sent to the mixer stage 5 through the diode 22 and a coupling capacitor 24. At this time, the local oscillator 4 does not operate and so the mixer stage 5 functions as an amplifier with the result that an IF signal derived from the super band section 2 is amplified and appears at the output terminal 53.
Finally, if a voltage is applied to a UHF band terminal 31, the UHF band section 3 operates, and an IF signal, which is produced when a UHF band signal is received by this section, is fed to the mixer stage 5 through a switching diode 32 and a coupling capacitor 34 in the same manner as in the aforementioned super band section 2. At this time, the local oscillator 4 does not operate and so the mixer stage 5 functions as an amplifier. In this manner, an IF signal produced from the section 3 appears at its output terminal 53 after subjected to an amplification operation.
It is to be noted that in each of the above-mentioned operations, a constant voltage is applied to a transistor 54 in the mixer stage 5 by a bias circuit made up from resistors 16 and 51, as a matter of course.
In the circuit of FIG. 1 described so far, it is necessary to alter the values of the coupling capacitors 14, 24 and 34 in order to control the gain of the mixer stage 5. If the bias voltage most suitable to the case where the stage 5 is used as a mixer as encountered in receiving a VHF band signal is applied unchanged to the transistor 54, then a deterioration in the distortion characteristic of the mixer stage will result when the stage 5 is operated as an amplifier as in a case where the super band section 2 or UHF band section 3 is operated. Conversely, if the bias voltage optimal for the case where the stage 5 is operated as an amplifier is applied to the transistor without varying its magnitude when the stage acts as a mixer, then the gain will decrease extremely. Consequently, a trade-off between the mixer operation and the amplifier operation has been imposed on the design of this circuit. Also undesirably, this kind of circuit has unwanted couplings between the sections 1 and 2 and between the sections 1 and 3.